Every marathon course is challenging for a different reason: New York's is known for the bridges that connect the city's five boroughs. Boston's is famous for its tough hills. And October's Marine Corps Marathon in Washington, DC is notorious for its uphill finish. As professional runners, we've learned a lot about how best to train for marathons, who runs them, why they're so popular — and which myths deserve debunking.

1. It's hard to secure a spot in a marathon.

The New York City Marathon is so popular that it has a lottery system to distribute entries. And some races, like Boston's, do require a qualifying time to enter. But even in Boston, it is possible to get a spot without one.

Runners who miss an entry deadline or who do not have the qualifying time needed can often join a race on a charity entry if they raise a minimum amount for a charitable organization. Such teams, which our foundation also sponsors, can build camaraderie and motivate runners because they're running for a cause, not just for themselves.

But for most races, all you need to do is simply register before the deadline. If a marathon is competitive, it may sell out in less than 24 hours. But for others, you can register even up to the day of the race, depending on the event's size.2. Runners should "carbo-load" by eating pasta the night before the race.

As much as runners enjoy any opportunity to binge on food guilt-free, the best way to "carbo-load" is to eat just 100 extra grams of carbohydrates, the equivalent of three bagels, spaced out throughout the day before the race.

Your muscles can store only a certain amount of carbohydrate at one time for race day. Anything beyond that is stored as fat, which no runner wants weighing him down.

In Sara's early running days, she once ate six bowls of oatmeal before a race in an attempt to carbo-load. After a massive stomach cramp hindered her performance, she realised that a last-minute carbohydrate dash was not effective!

So instead of piling on the pasta the night before, it's better to graze on snacks with extra carbohydrates between meals. And taking in protein and healthy fat with your carbs — rather than having a carbohydrate-only meal such as spaghetti with marinara sauce — helps slow the absorption of the carbohydrates. That way, more carbs become stored fuel for the next day's race, and fewer turn to fat.

3. You have to be very fit to run a marathon.

One of the things we love about marathons is the diversity of the runners. After all, there aren't many events where you see tens of thousands of people of all ages, shapes and ethnicities participating together.

Watch a marathon from the sidelines and you'll see that, for most runners, becoming physically fit is still a work in progress — and the race is often a milestone in that journey.

We've known some runners who haven't even been exercising, let alone training for a marathon, who have been able to complete a race. This is clearly not advised — and if you ask one of them when they finish or wake up the next day, they will tell you why.

Some of the marathoners who have inspired us most are those who, since they are not yet fit enough to run the race, walk the entire thing. We see them walking the final miles, determined to finish, while the course is being dismantled around them.

4. Everyone hits the wall at Mile 20.

If runners believe this, it can become a self-fulfilling prophecy — but it doesn't need to be.

"Hitting the wall" is when your muscles begin to cramp or stop working because of a loss of glycogen, the fuel stored in muscles. Proper hydration and caloric intake, such as eight ounces of a sports-performance drink (such as Cytomax) every three miles along the course and during training runs, can give your muscles the glycogen they need to continue working, despite fatigue.

Another way you can lose glycogen is by starting out too hard and burning it up in the early stages of a marathon. As long as you train consistently for the race and stick to the pace to which you trained, you should finish strong.

The only time Ryan came close to hitting the wall was in the last stages of the 2012 Olympic Trials Marathon. He had an upset stomach from taking strong anti-inflammatory medications for a foot injury. Because his stomach didn't want any Cytomax drinks and he wasn't finishing his bottles, he started getting dizzy and his muscles fatigued in the final miles. Fortunately, however, he was able to maintain second place and secure his second Olympic berth.

5. US-born marathoners will never beat the Kenyans.

As a whole, Kenya clearly has the most marathon talent compared with any other country in the world, based on annual top race times. However, in the past three Olympic Games, only one of the six gold medalists in the men's and women's marathons was Kenyan. The other champions were from Italy, Japan, Romania, Ethiopia and Uganda.

There have also been winners, including Americans, in the World Marathon Majors — a series of five annual races in Berlin, Boston, Chicago, London and New York - who have beaten a strong contingent of Kenyan runners.

Ryan has placed as high as third at one of the World Marathon Majors, ahead of many talented Kenyans, so it is possible to compete with even the best Kenyan runners. He hopes to learn more about what makes the Kenyans so successful when he goes to Kenya's Rift Valley to train this winter.Ryan Hall and Sara Hall are professional runners for Asics and co-founders of the Hall Steps Foundation, which brings health care to underserved communities worldwide. Ryan is a two-time Olympic marathoner and holds the fastest American marathon time at 2:04:58. Sara is the Pan American gold medalist in the steeplechase.